The news is reporting that our area is due to be hit this week by the heat wave
that's been cooking the west coast. This is not great news, as our weather was
already, as the line from Biloxi Blues put it, "Africa hot."

No, with that kind of system you would need at least 500W, and I'd go 550-700W for safety

Yeah 550W seems to be adequate. Though I read (somewhere) that the Conroe systems are more worried about the 'quality' of the power provided to them - i.e. a stable X volts, not +/- 10% like some of the crappy PSUs give out. But I guess any decent brand name PSU (I think I've always had Enermax) will be fine.